In recent years, an automatic-control-type air conditioning system is widely used in motor vehicles. The automatic-control-type air conditioning system detects a vehicle indoor temperature, a vehicle outdoor temperature, a vehicle indoor humidity, a vehicle outdoor humidity, a solar radiation amount and so forth. Using the detection data thus obtained, the air conditioning system automatically controls a vehicle indoor temperature, thereby keeping a vehicle indoor environment pleasant at all times.
The automatic-control-type air conditioning system includes a defogging device that, when a fog is generated on a windowpane of a motor vehicle, automatically removes the fog adhering to the windowpane.
As shown in FIG. 1, the defogging device includes a defog sensor 1 for sensing a windowpane relative humidity and a control unit 9 for controlling a defrost door 3, an intake door 5, a air conditioner compressor 7 and a blower 8 depending on the windowpane relative humidity inputted from the defog sensor 1.
The defog sensor 1 includes a temperature sensing unit 1a for sensing a windowpane temperature, a humidity sensing unit 1b for sensing a windowpane humidity, and a microcomputer 1c for calculating a windowpane relative humidity by processing the windowpane temperature sensed by the temperature sensing unit 1a and the windowpane humidity sensed by the humidity sensing unit 1b. 
As illustrated in FIG. 2, the defog sensor 1 is arranged within a vehicle room and is attached to an upper central portion of a windowpane G. The defog sensor 1 thus attached makes physical contact with the upper central portion of the windowpane G to sense a humidity and a temperature of the windowpane G. By processing the humidity and the temperature thus sensed, the defog sensor 1 calculates a relative humidity of the windowpane G.
If the windowpane relative humidity inputted from the defog sensor 1 is equal to or higher than a reference humidity, the control unit 9 determines that a fog has been generated on the windowpane. Based on this determination, the control unit 9 comes into a defogging mode in which the defrost door 3 is opened at a predetermined opening degree, the intake door 5 is converted to an outdoor air mode, and the compressor 7 and the blower 9 are operated.
As a result, a fresh external air is cooled into a cold air while passing through an evaporator 7a. The cold air is discharged toward the windowpane through a defrost vent 3a. The cold air thus discharged can remove a fog adhering to the windowpane.
In the conventional defogging device mentioned above, the defog sensor 1 is installed in the upper central portion of the windowpane G. This poses a shortcoming in that the defog sensor 1 can merely sense only the relative humidity in the upper central portion of the windowpane G which does not assist in securing a driver's view field.
Due to this shortcoming, there is a problem in that the generation or non-generation of a fog in the windowpane G has to be determined by relying on the windowpane portion which does not assist in securing a driver's view field.
Accordingly, the defogging device is unnecessarily operated even when a fog is generated only in the windowpane portion which does not assist in securing a driver's view field. This leads to unnecessary energy consumption, as a result of which the fuel efficiency of a motor vehicle is reduced.